The present invention relates generally to a process and apparatus for producing a dry-pressed molding from a particulate or granular molding material, such as a ceramic molding composition. In particular the invention relates to a process of the type in which the molding composition is introduced into a loading cavity where it is premolded, then the premolded molding composition is pressed into the final shape of the molding by means of mutually oppositely moving halves of a molding press, the molding surface of each of which mold halves is substantially rigid or formed by an elastomeric layer which is supported during the final molding process in such a way as to be dimensionally rigid.
A process of this general type is known. In the known process, the loading cavity is formed by the two halves of a molding press which also constitute the final press in which the premolding is pressed into the final molding. In one known process for producing a round flat plate the halves of the molding press are arranged in such a way that the axis of the plate is essentially horizontal. The particulate or granular mold charge is introduced at an upper edge of the mold cavity and flows under the influence of gravity vertically downwards into the loading cavity. Once the loading cavity has been filled in this way the mold halves are moved towards each other and the molding receives its final shape. This known method is suitable for producing moldings where there are no large differences in wall thickness.
If the molding to be produced varies widely in wall thickness, as is the case for example with pieces of crockery with a pronounced base or a steep rim, it is difficult to obtain even an approximately uniform degree of compression within the molding because the mold halves, which are rigid, move towards each other during the pressing operation by the same distance across the entire area perpendicular to the direction of the pressing movement. If this pressing movement is assumed for example to be 1 cm, this means that at a point where the depth of the loading cavity is 2 cm (in the direction of pressing movement) before the molding, the linear compression ratio will be 2:1. By contrast, at a point where the depth of the same loading cavity is 4 cm in the pressing direction, the linear compression ratio will only be 4:3. Even though the ability of the molding composition to flow transversely of the pressing direction will have a certain averaging effect on the compression ratio between zones of differing loading cavity depths, it is nonetheless unavoidable that there will be differences in the degree of compression in the finished molding when the pressing is complete. On firing such a molding these differences in the degree of compression are the cause of uncontrollable deformations.
It is known to use so-called isostatic molding presses to obtain approximately consistent compression ratios. In isostatic molding presses at least one of the mold halves is lined with a resilient press membrane which, at rest, bears against a rigid supporting surface. The gap between the membrane and the rigid supporting surface is connected to a pressure fluid supply source so that, as the pressure is applied, the press membrane moves away from the supporting surface, producing an almost uniform compression in the molding composition as a result of the fact that the press membrane moves different distances at different zones, where the loading cavity has different depths. Isostatic molds and the associated presses and necessary hydraulic pressure sources are technically complicated and costly. Moreover, considerable skill is necessary to determine the required shape of the supporting areas of the press membrane so that the surface of the membrane facing into the mold cavity takes on a given desired shape during the pressing operation.